1. Field of the Invention
The present invention relates to a method for annealing metal parts, in particular sheets, in an annealing chamber of an industrial furnace under a protective-gas atmosphere, wherein the annealing comprises a heating phase, a soaking phase following thereafter and a cooling phase following the two previous phases. The heating of the annealing charge during the heating phase can be according to a pre-set temperature profile over the period of time. The same applies to the cooling of the charge during the cooling phase. During the soaking phase the charge is substantially kept at a constant temperature.
2. Prior Art
Methods of this kind are used particularly for bright annealing cold-rolled steel sheets for the car industry. It is known to use a hydrogen/nitrogen mixture as the protective gas, the hydrogen content of the mixture being greater than the nitrogen content. The mixing ratio is, for example, 75% H.sub.2 to 25% N.sub.2 and remains constant throughout the entire annealing process. Instead of nitrogen, argon or another inert gas can also be used.
The percentage of hydrogen in the protective gas produces very good heat transfer conditions and also ensures that contaminants are removed from the metal parts. A layer of oil forms during the cold-rolling of sheets which is removed in this manner during bright annealing.
When steels with readily oxidisable alloy elements are annealed, so-called annealing edges of differing widths form at the edges of the steel strips. These annealing edges are formed by oxide layers of the readily oxidisable alloy elements. They make the further treatment of the steel sheets more difficult even if they cannot be seen with the naked eye, as, for example, is the case with aluminium oxide. Furthermore, readily oxidisable alloy elements include not only aluminium and titanium but also above all manganese, silicon and chromium.